1. Field of the Invention
The present invention relates to a release member for use in the production of multi-layer printed wiring boards.
The present application is based on Japanese Patent Application No. Hei. 11-199138, which is incorporated herein by reference.
2. Description of the Related Art
Multi-layer printed wiring boards are manufactured by first laminating wiring boards with prepregs and other hot melt adhesive sheets being interposed and then pressing them under heat so that they are made integral by the adhesive sheets. If the wiring boards to be laminated have holes, part of the molten adhesive sheet will get into the holes to cause a certain inconvenience such as closure.
FIGS. 4A to 4C show steps in a conventional process for manufacturing a multi-layer printed wiring board. As shown in FIG. 4A, a copper foil 1" fitted with an adhesive film 13' and having a hole 14' is made integral with a wiring board 1' by bonding with the adhesive film 13'. Then, as shown in FIG. 4C, a predetermined conductor pattern is formed in the copper foil 1" and electrical continuity is established between the conductor pattern and an internal conductor line a' in the wiring board by means of copper plate b'. A problem with this process is that on account of the pressure and heat applied to make the copper foil integral with the wiring board, part of the molten adhesive film 13' sometimes gets into the hole 14' to break the conductor path.
In order to deal with this situation, it has been proposed that the perforated copper foil with an adhesive film be overlaid with a release member F' which, as shown in FIG. 4B, consists of a stripping film A' such as a fluoroplastic film, a thermoplastic resin layer C' such as a polyethylene layer, and a stripping film B'; when pressed under heating, the thermoplastic resin layer C' deforms to fill the hole 14' with the lower stripping film A' being at the bottom, thereby preventing the molten adhesive 13' from flowing into the hole 14'.
As the thermoplastic resin layer C' deforms, the lower stripping film A' develops resistance but this is extremely small since the depth of the hole 14' in the copper foil 1" is so small (10-50 .mu.m) that the strain (elongation) of the film A' is virtually zero.
However, if the hole is several thousand micrometers deep, the release member has such a great resistance that the lower stripping film works very effectively to prevent the molten thermoplastic resin to flow into the hole, making it difficult for the resin to realize complete closure of the hole; as a result, complete prevention of the adhesive from getting into the hole becomes practically impossible.
Therefore, the conventional release member has been unsuitable for use in laminating an increased number of wiring boards and making them integral to produce a multi-layer printing wiring board having a deep IC chip accommodating cavity as shown in FIG. 2.
Another difficulty with the conventional release member is that gas evolves from the stripping film in contact with the side where holes are open, potentially causing a practical problem depending upon the structure of the multi-layer printed wiring board.